Explicit science instruction
An ionic formula gives the simplest whole-number ratio of ions needed for neutrality. Subscripts apply to the species they follow; parentheses preserve a polyatomic ion when more than one copy is required.
A reliable method is to calculate total positive and negative charge separately, then reduce the ratio if a common factor remains.
The cross-over shortcut can produce a candidate formula, but a charge audit is the scientific check that confirms it.
Vocabulary: formula unit; subscript; polyatomic ion; charge audit; empirical ratio.
Worked example with reasoning
For Al³⁺ and O²⁻, the least common charge magnitude is 6: two Al³⁺ ions contribute +6 and three O²⁻ ions contribute −6, so the formula is Al₂O₃.
Guided question
Construct the formula for Ca²⁺ and PO₄³⁻ and show the total positive and negative charge.
Independent work
- Build formulas for Na⁺/S²⁻, Mg²⁺/N³⁻, Al³⁺/Cl⁻, and NH₄⁺/SO₄²⁻.
- Write a charge audit beside every formula.
- Identify one formula that can be reduced and explain why empirical formulas use the simplest ratio.
Misconception check
Subscripts are not ionic charges. Charges describe individual ions; subscripts describe the count ratio in the formula unit.
Formative understanding check
Why is Ca₃(PO₄)₂ neutral?
Reveal response
Three Ca²⁺ ions contribute +6 and two PO₄³⁻ ions contribute −6.
Public self-check only.
Mentor evidence conference
Ask Archaemenes to challenge your particle, charge, and evidence reasoning before you finalize the week record.
Teacher guidance
Foundation: Use color-coded charge blocks.
Core: Construct and audit formulas including polyatomic ions.
Extended: Reverse-engineer likely ion charges from a neutral empirical formula and state ambiguity where multiple oxidation states are possible.
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